The invention refers to a control system for controlling the outer diameter of a strand of plastic material, particularly of a cable or the like.
Usually, cables are manufactured in that a conductor is moved through an extruder which applies an insulating layer onto the conductor, the material of the insulating layer being suited to be vulcanized or cross-linked. After the extrusion, the cable or the strand is conveyed to a vulcanization or cross-linking path and thereafter cooled. In order to achieve a predetermined insulation strength, the layer extruded onto the cable has to have a minimum thickness. On the other side, the diameter of the layer is to be selected only as large as necessary to save material and weight. For this reason, it is necessary to continuously monitor the diameter of the cable or the strand and to carry out a respective control if a predetermined value is exceeded or has not come up.
For the above reason, it is a necessity to continuously measure the outer diameter of the cable. The diameter can be changed by changing the conveying speed of the cable and/or the rotational speed of the extruder. Conventional control means compare the measured diameter with a reference value and form the difference of the signals which is transmitted to a control unit to change the conveying speed of the extruder line or the rotational speed of the extruder until the reference value has been reached. If the dead time of the system to be controlled is relatively small, i.e. if the distance between the extruder and the device measuring the diameter is relatively small, the behaviour of the control system can be satisfactorily adjusted to a predetermined diameter value. A disadvantage with respect to the conventional control is the fact that a satisfactory balancing can be achieved only for a specific diameter value. If the cable to be manufactured has a smaller diameter, the control works more slowly. If a larger diameter is to be manufactured, the control works more rapidly and therefore may become unstable.